Die Gewinner der SPG Preise 2021

Quantum correlations and communications

Quantum theory makes predictions of correlations that cannot be explained by classical theories. Recent decades have explored these correlations from several points of view: foundationally understanding quantum theory, quantum information science and its implementation in quantum technologies. This PhD dissertation contributes to aspects of this research program mainly in the following ways.
1) Certification of quantum devices. Simple communication experiments are proposed for certification of a broad fauna of quantum devices subject only small assumptions. Methods are also developed for the strongest form quantum certification targeting high-dimensional quantum measurements.
2) Communication complexity. Quantum correlations can boost the efficiency of communication. This relationship is explored for different types of quantum resources and methods are developed to characterize of quantum advantages. Also, it is proposed to decouple the notion of a quantum bit from the idea of a two-level system. This alternative avenue to informationally restricted correlations is found to qualitatively go beyond traditional qubits.
3) Bell nonlocality in networks. Bell’s theorem is a milestone of quantum theory. Recently this research program has expanded into more sophisticated scenarios corresponding to networks. Little is still known about this and several exploratory works into this topic are presented, revealing new types of quantum nonlocality.
4) Autonomous entanglement generation. It is investigated whether strong entanglement can be generated from a thermal machine that only harvests spontaneous environmental interactions. It is found that such minimal machines can probabilistically produce the strongest form of high-dimensional and multi-qubit entanglement.

New fermions with large topological charges in chiral topological semimetals

Chiral topological semimetals are a new class of topological matter that host chiral multifold fermions in a chiral crystal structure. These new fermionic quasiparticles can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. Their large topological charge has been predicted to give rise to unusual phenomena, such as giant quantized photocurrents, long fermi-arc surface states, unusual magnetotransport signatures, new spin-orbit torques, or unconventional and topological superconductivity. Whilst there have been many theoretical predictions related to multifold fermions in previous years, they have so far remained elusive in experiments.

Here I will report the experimental observation of multifold fermions in a chiral topological semimetal. Using angle-resolved photoelectron spectroscopy, we directly visualize their long fermi-arc surface states and resolve a band splitting that indicates that they carry the largest topological charge that can be realized for quasiparticles in any material. We are also able to show experimentally that there is a direct relationship between the handedness of the crystal structure and the electronic chirality (i.e. the Chern number sign) of the multifold fermions, which indicates that structural chirality can be used as a control parameter to manipulate phenomena that are sensitive to the electronic chirality, such as the direction of topological photocurrents. I will then also present our latest experimental results about new directions in the field of chiral topological semimetals.

[1] N. B. M. Schröter et al., Nat. Phys. 15, 759–765 (2019).
[2] N. B. M. Schröter et al., Science 369, 179 (2020).

Development of an advanced hybrid MOSFET/tunnel FET platform

High power consumption represents a major bottleneck for conventional transistor technologies (MOSFETs), due to the inability of further reducing supply voltage while simultaneously limiting the off-state leakage current. This limitation can be overcome by Tunnel FETs, a novel transistor concept based on the quantum-mechanical band-to-band tunneling, but the best TFET demonstrators so far are based on vertical nanowire approaches which are not compatible with advanced IC technology. During my PhD research, I demonstrated the first hybrid technology platform combining III-V Tunnel FETs and MOSFETs with a scalable process and suitable for large-scale semiconductor manufacturing [1]. Such low-power technology platform paves the way to future energy efficient electronics, with the ultimate goal to reduce the carbon footprint of the ICT industry. The demonstrated platform exploits the synergies between the world of Tunnel FETs and of III-V MOSFETs [2], as it allows to implement hybrid logic blocks tailored to the unique specifics of each device. Tunnel FETs provide lower leakage and good performance at low voltages levels, MOSFETs are faster (at same dimension and bias) and provide greater current drive. The developed fabrication flow is identical for both devices except for a single masking and epitaxy step, opening up for manufacturing of truly hybrid logic blocks. Unlike a MOSFET, the TFET is an asymmetric device with different materials in the source and drain regions. Key to achieving the reported performance is the invention of a selective source approach, where the GaAsSb source is grown right before the replacement gate process, resulting in self-alignment of the gate. The fabricated TFETs show a subthreshold swing of 47 mV/decade, outperforming the state-of-the-art with a superior device platform and unmatched scaled device dimensions.

[1] C. Convertino et al., “Hybrid III-V tunnel FET and MOSFET technology platform integrated on silicon,” Nature Electronics (2021). https://doi.org/10.1038/s41928-020-00531-3
[2] C. Convertino et al., “InGaAs-on-Insulator FinFETs with Reduced Off-Current and Record Performance,” in IEEE International Electron Devices Meeting (IEDM), pp. 39.2.1-39.2.4, 2018. https://ieeexplore.ieee.org/document/8614640

Discrete-time signal processing with NV centers

In 1950, Erwin L. Hahn reported a seminal experiment on detecting the oscillating magnetization of atomic nuclei following excitation by a short burst of radio-frequency radiation. This so-called “free induction decay” (FID) signal initiated the field of modern NMR spectroscopy, which has become a standard tool in chemical analytics and medical MRI tomography.

While NMR and MRI operate on large sample volumes (including humans), it has been unclear from a basic physics standpoint whether FID detection can be extended down to a single nuclear spin, or whether this is inhibited by the laws of quantum mechanics. Until recently, this question has been mainly theoretical, because the magnetic signals of single nuclear spins have been far too week for existing magnetic detectors. However, recent advances with quantum defect spins in diamond, most notably the nitrogen-vacancy (NV) center, have brought single nuclear spin detection within reach.

NV centers are atomic-scale defects that are sensitive, among others, to external magnetic fields via dipolar interactions. In recent years, they have attracted considerable attention for their potential as highly sensitive magnetic probes. Owing to their inherent quantum nature, they are extremely sensitive to perturbations, and thanks to their atomic-scale size they can be brought in very close proximity to a target.

In our work, we used a spectroscopy method based on sequential weak measurements (also known as quantum non-demolition measurements) to detect the FID signal of a single carbon-13 nuclear spin [1]. We showed that such measurements mitigate the unwanted quantum back-action, and provide a number of further advantages, including a large frequency bandwidth and possibility of efficient Fourier NMR methods. Building on this experiment, we further showed that single nuclear spin NMR can be extended to image large nuclear spin clusters with three-dimensional atomic resolution. As a proof-of-principle, we demonstrated the detection of up to 29 carbon-13 nuclear spins in diamond, and showed how, by applying information-criteria principles to the detected signals, the three-dimensional atomic positions of nuclei in the diamond lattice can be recovered [2]. Next steps include functionalizing diamond surfaces with single target molecules (like proteins) and to apply the single-spin NMR techniques to structural imaging and monitoring of chemical surface reactions.

[1] K. S. Cujia, J. M. Boss, K. Herb, J. Zopes and C. L. Degen. Tracking the precession of single nuclear spins by weak measurements. Nature 571, 230 - 233 (2019).
[2] K. S. Cujia, K. Herb, J. Zopes, J. M. Abendroth and C. L. Degen. Parallel detection and spatial mapping of large nuclear-spin clusters in diamond. arXiv:2103.10669 (2021).

Neural Network Quantum States

Over the past few years, Artificial neural networks (ANN) have led to numerous breakthroughs from improving language translations to beating the best players in Chess and Go. In the field of condensed matter physics, ANNs have also been recently introduced as a general ansatz known as neural network quantum states (NQS) to represent many-body wavefunctions. In conjunction with variational Monte Carlo calculations, this ansatz has been applied to find Hamiltonian ground states and their energies [1]. We have extended upon this approach to study excited states, a central task in several many-body quantum calculations. By using a Gram-Schmidt type orthogonalisation procedure, we were able to obtain the first excited state of a system. In addition, by incorporating spatial symmetries further spectral properties could be addressed.

Furthermore, we have also applied the methods developed to the J1-J2 model on the square lattice [2]. Using deep convolutional networks, we showed that the NQS are able to achieve competitive results with respect to other state-of-the-art approaches such as density matrix renormalisation group, across the full phase diagram.

[1] K. Choo et al. “Symmetries and Many-Body Excitations with Neural-Network Quantum States“, Phys. Rev. Lett. 121, 167204 (2018)
[2] K. Choo et al. “Two-dimensional frustrated J1-J2 model studied with neural network quantum states”, Phys. Rev. B 100, 125124 (2019)